Due to high system efficiency over a wide range of speed, multiphase switched reluctance motors (SRMs) are favored for their variable-speed drive applications. A conventional SRM includes salient rotor and stator poles, as shown in FIG. 1. Such motors typically have a fixed stator structure comprising one or more phase windings, and a rotor structure. As shown, each pair of diametrically opposite stator pole windings is connected in series or in parallel to form an independent phase winding of the multiphase SRM. Direct current is selectively switched to pass through the phase windings. Resultant electromagnetic fields induced by the windings interact with the fixed fields of the rotor in a manner resulting in a rotary force or torque which causes the rotor to rotate relative to the stator.
Illustrated in FIG. 2, when the stator and rotor poles are aligned, maximum inductance exists. Applying the definition of torque ##EQU1##
and noting the relationship between inductance, angular position and phase in FIG. 3, when the poles are aligned, the change of inductance, dL, is positive. Thus, torque, T, is positive. In order to maintain positive torque, however, it is necessary to switch the applied current off at some reference angle .theta..sub.ref just prior to the change of inductance dL of the phase becoming negative which corresponds to the phase at an unaligned position. Thus, switching current from one phase winding to the next in a predetermined sequence that is synchronized with the angular position of the rotor continuously generates positive torque. As such, SRMs require angular rotor position sensing devices to determine the position of the rotor and thereby, maintain positive torque of the rotor.
In conventional SRMs, a shaft angle transducer, such as an encoder or a resolver, generates a rotor position signal and a controller reads this rotor position signal. In an effort to improve reliability while reducing size and cost, various approaches have been previously proposed to eliminate the shaft position sensor by determining the reference commutation angle. These approaches implement indirect rotor position sensing by monitoring terminal voltages and currents of the motor.
One approach is disclosed in U.S. Pat. No. 4,959,596, issued to S. R. MacMinn, et al., on Sep. 25, 1990 which patent is incorporated by reference herein. As disclosed, a method of indirect motor position sensing involves applying voltage sensing pulses to one unenergized phase. The result is a change in phase current which is proportional to the instantaneous value of the phase inductance. Proper commutation time is determined by comparing the change in phase current to a reference current, thereby synchronizing phase excitation to rotor position. Phase excitation can be advanced or retarded by decreasing or increasing the threshold, respectively. Due to the unavailability of inactive phases during higher speeds, this commutation method which makes use of the inactive phases of the SRM are limited to low speeds. Furthermore, although current and torque levels are relatively small in an inactive phase, they will contributed to a loss in SRM efficiency.
Another such approach is disclosed in U.S. Pat. No. 5,140,243, issued to J. P. Lyons, et al., on Sep. 25, 1990 which patent is incorporated by reference herein. As disclosed, a method of indirect motor position sensing involves using a flux-current map of a given SRM, such as the one illustrated in FIG. 2. Utilizing this flux-current map, measured phase voltage, phase current and phase resistance and estimated flux provided necessary data to determine a reference angle. Comparison of the estimated phase flux to the reference flux is the basis for commutating the motor. The disadvantage of this approach is that the flux-current characteristics of a motor are not readily known; thereby, requiring costly calibration measurements. Additionally, these characteristics exhibit change over time, requiring recalibration of the SRM. Therefore, this commutation method is costly.
Although the above-cited patent advantageously provides a method for indirectly determining rotor position so that a conventional rotor position sensor is not required, it is desirable to provide a method which does not require the need for prior knowledge of the flux-current characteristics of the SRM.
A control system and method for a multiphase switched reluctance motor (SRM) provides commutation of the motor operable at high speeds, requiring no rotor position sensor not detailed prior knowledge of the SRM magnetic characteristics. This commutation method and system includes two routines: a calibration routine and a commutation routine. The calibration routine is a self-training calibration routine to determine the flux-current characteristics of each phase in an aligned position. During this calibration routine, voltage sensing pulses of current applied to an active phase (i.e. one producing torque) create a change in phase current which is inversely proportional to the instantaneous value of the phase inductance. Using the integral form of Faraday's law, each pulse of current provides the appropriate variables to determine phase flux. Subsequent interpolation of the data to fit a curve provides the necessary data for deriving flux-current characteristics at a reference angle for commutating the motor. During the commutation routine, a commutation algorithm commutates the SRM by measuring the flux in an active phase and comparing the flux to one approximated for the reference angle.
The method of the invention is particularly well-suited for relatively heavy duty loading applications, such as a fan.